Sludge management is a growing problem for water utilities. Because alum constitutes thirty to fifty percent of the total clarifier sludge, recovering it would significantly reduce the amount of solid waste that water utilities must dispose. Moreover, recovered alum can be reused as a coagulant, thereby reducing water treatment operating costs.
Aluminum is not included on the list of priority pollutants issued by the U.S. Environmental Protection Agency. Nevertheless, the toxic effect of free and complexed aluminum species on various aquatic life, including fish and benthic organisms, has been the focus of recent investigation. Consequently, although clarifier sludge is not a hazardous waste, its high aluminum content will prompt closer scrutiny in the future of clarifier sludge disposal.
Many of the sludge-producing water treatment plants in the United States are turbidity removal plants, which account for about seventy percent of all water treated. Water treatment plants use coagulants such as alum and ferric sulfate to remove the turbidity in raw water and produce coagulant sludges containing aluminum, magnesium, and iron hydroxides. The coagulant, an acidic sulfate, must be disposed or recovered. About twenty-five percent of the waste producing plants are water softening plants. They account for about twenty-five percent of all water treated. The problem with sludges is similar in other countries.
The need to eliminate sludge discharges has imposed both an economic and a technological problem on the water supply industry. Recently, sludge processing plants have been constructed to treat coagulant. Most disposal practices involve one or more of four alternatives. Co-disposal of the coagulant with sewage sludge at a waste water treatment plant is one alternative; co-disposal often is impractical because sewer access may be unavailable. Lagooning with and without natural freezing, which requires ultimate disposal of the residue in the future, may be a viable alternative at plants where large tracts of inexpensive land are available. Mechanical dewatering with landfills of residue is a third alternative; it is slow and expensive, especially for plants with a water treatment capacity of less than fifty million gallons per day, and undesirable because land disposal of solid wastes has emerged as a national problem (minimizing the volume of solid wastes is a top environmental priority). Finally, only coagulant recovery, of all the potential methods of sludge treatment, offers the ability to reduce the size of the facilities required and to provide a lower life cycle cost.
Several processes have been developed to recover coagulants, particularly the alum produced from aluminum hydroxide which is dispersed in an aqueous sludge resulting from precipitation or coagulation with alum in potable water clarification. They all have the basic disadvantage that the recovering means can be regenerated and reused only with great difficulty and cost. One process involves a substantial lowering of pH to solubilize the metal ion in a metal hydroxide precipitate by adding acid. See U.S. Pat. No. 3,959,133 issued to Fulton.
In the acid digestion process, sludge, usually from settling basins and filter backwashes, is collected in an equalization tank and subsequently thickened by gravity. The supernatant water from the tank is returned to the plant raw water intake. Acid (typically sulfuric acid) is added to the thickened sludge at a concentration determined by the amount of metal hydroxide in the sludge and the desired level of recovery. The dissolved aluminum, in the form of liquid alum, is separated from the residual solids by a gravity separator and returned to storage for reuse while the residual sludge is disposed of by landfills after neutralization.
The stoichiometry of the reaction, by which clarifier sludge is sufficiently acidified with sulfuric acid and insoluble aluminum hydroxide is dissolved in the form of dilute liquid alum, can be written as follows: ##STR1## The above equation summarizes the underlying concept of the acid digestion process, which was implemented at the water treatment plant of the city of Durham, N.C. with encouraging results.
The acid process for alum recovery has a number of shortcomings which may preclude water utilities from adopting and applying it. One potentially serious problem is that certain impurities may accumulate in the recovered and recycled alum. The process is nonselective; along with alum, it also recovers other impurities which are soluble under highly acidic conditions or which exist as colloids. If this occurs and the recovered alum is recycled for water treatment, the potable water may suffer degradation. Consequently, the acid process may not be used in areas where such impurities present problems.
Potential impurities include those which can be converted to a soluble form in the acidulation process, such as iron, manganese, chromium, other metals, and those from impure sulfuric acid. The concentration of heavy metals (copper, lead, cadmium, and others) in the clarifier sludge is normally low. That concentration may warrant concern, however, if the sludge is recycled through recovered alum. For example, the manganese concentration is significant in raw water in many areas and is likely to be very high in the recovered alum for the acid digestion process. Tests run at the Durham plant confirms that, in recovered alum, the manganese concentration is as high as 127 mg/g of aluminum. Contrast commercial alum, in which manganese is practically absent. This problem may occur for any element present in ionic or colloidal form in the raw water but which has a low solubility and a high settling rate in the clarifier. Such an element would tend to concentrate in the recovered alum during the acid digestion process.
Other impurities which the acid digestion process typically recovers along with alum include a wide variety of naturally occurring organic materials (humates and fulvates). Should the recovered alum be reused as a coagulant, the concentration of organic matter (trihalomethane precursors) and the potential to form trihalomethane in the treated water would tend to increase in the clarified water.
Researchers have tried extracting solvents using liquid ion exchangers for selective alum recovery. This process uses organic solvents to recover pure, concentrated alum from sludge by liquid ion exchange. The liquid ion exchange is a process step performed after acid is added to the alum sludge and is used to eliminate acid soluble impurities from the recovered alum. Such a process is operationally complex, however, and solvent carryover or entrainment into the recovered alum is unavoidable without incorporating an additional treatment step. Moreover, the economics of liquid-ion exchange have yet to be proven.
Chelating agents are useful both in theoretical and applied chemistry and related fields. Common uses include both qualitative and quantitative analyses, water softening, catalysis, and solution clarification. Chelating agents are particularly useful in treating aqueous effluents to remove metallic impurities.
Typically, chelating agents are used in either liquid ion exchange systems (see U.S. Pat. No. 4,334,999 issued to Cornwell) or packed columns (see W. Waitz, Ion Exchange in Heavy Metals Removal and Recovery, Amber-Hi-Lites, 162 (Rohm and Haas Co., Philadelphia, Pa.); T. Roy, Chelating Polymers: Their Properties and Applications, Master's Thesis, Lehigh University, Bethlehem, Pa. (1989)). In the former, an organic solution containing the chelating agent is placed in intimate contact with the aqueous solution. Metallic ions are removed when the two phases are separated. Unfortunately, a clean separation of the phases is difficult to achieve; some loss of organic solvent invariably results. In the latter, the aqueous solution passes through a column or "bed" of solid chelating resin, which is often mixed with a solid, inert carrier. Such a process requires extensive pretreatment, however, and cannot handle sludges or slurries which have high solids contents.
The preceding discussion stresses the need for a cyclic, selective, easy-to-operate, efficient, economical coagulant recovery process for water utilities. An object of the present invention is, therefore, to provide an improved and different process for selective removal of coagulant (especially alum) from waste water clarifier sludge without encountering the shortcomings associated with the processes discussed above. A related object is to provide a process in which a metal ion can be extracted directly from an insoluble metal hydroxide or salt dispersed in an aqueous phase containing five to ten percent suspended solids. Further, it is an object of the present invention to provide a process which does not significantly accumulate organic material or heavy metals in the liquid coagulant recovered--as occurs in the acid digestion process. Still further it is an object of the present invention to provide a process which can be operated with relative ease when compared to existing processes.
In addition, it is an object of the present invention to provide a system which can be regenerated and used continuously for long periods of time. Another object is to provide a high surface area system which is compatible with sludge, thereby avoiding the need to pretreat the sludge.